test(telemetry): de-flake FeatureFlagCache placeholder test (network seam)

The "fetchFeatureFlag should return false as placeholder implementation"
test called the real fetchFeatureFlag, which makes an HTTP call via
fetchWithRetry (10s timeout) to a bogus host. Under mocha's 2s default it
only passed when the DNS failure happened to resolve quickly — flaky
across runners and Node versions (it timed out on Node 14/16/18 in CI,
fail-fast-canceling the rest of the matrix; Node 20 passed).

Stub the fetchWithRetry network seam so the test deterministically
exercises the behavior under test (fetchFeatureFlag resolves to false)
with no real network call. No production code change.

Co-authored-by: Isaac
Signed-off-by: Madhavendra Rathore <madhavendra.rathore@databricks.com>

Author: msrathore-db

tests/unit/telemetry/FeatureFlagCache.test.ts

@@ -257,9 +257,19 @@ describe('FeatureFlagCache', () => {
       const cache = new FeatureFlagCache(context);
       const host = 'test-host.databricks.com';
 
+      // Stub the network seam so the test is deterministic. The real
+      // `fetchWithRetry` makes an HTTP call with a 10s timeout to the
+      // (bogus) host; under mocha's 2s default this passed only when the
+      // DNS failure happened to resolve quickly — flaky across runners /
+      // Node versions (it timed out on Node 14/16/18 in CI). The behavior
+      // under test is just that `fetchFeatureFlag` resolves to `false`.
+      const fetchStub = sinon.stub(cache as any, 'fetchWithRetry').rejects(new Error('network disabled in test'));
+
       // Access private method through any cast
       const result = await (cache as any).fetchFeatureFlag(host);
       expect(result).to.be.false;
+
+      fetchStub.restore();
     });
   });